In a situation where a device such as an amplifier receives a large amplitude signal which would make node voltage of the device higher than its withstand voltage, the amplitude of the signal is conventionally limited by a clipping circuit which uses fixed forward voltage of a diode to protect the device.
There has also been used a clipping circuit which uses avalanche breakdown caused by inserting a device different in withstand voltage from the main device in parallel with a main device used for amplification and the like.
In the above clipping circuit using the forward voltage of diode, the number of the diodes to be connected in series is adjusted to control clipping voltage. The forward voltage of a silicon diode is generally 0.6 V, and this voltage is used as a minimum adjustment unit. However, in miniaturization of CMOS process and the like, gate withstand voltage is lowered to about 1 V, which makes it necessary to control a clipping amount in several dozen mV order. Accordingly, the above diode clipping scheme is not sufficient in control resolution of the clipping amount and is therefore not applicable. Since MOSFET threshold values are not correlated with manufacturing variation of forward voltage of diodes, a compensating circuit needs to be separately provided in order to maintain the clipping amount constant regardless of the variation.
Further, the above clipping circuit using the avalanche breakdown has few options regarding the withstand voltage of transistors which can be manufactured on the same wafer by such process as CMOS process. When the withstand voltage of a high withstand voltage transistor is close to or equal to the withstand voltage of a low withstand voltage transistors, a clipping effect cannot be acquired.